Fig. 4

Approximation of the low-frequency sub-signal at three decomposition levels in the Haar wavelet transformation. a The original signal f(t). b The low-frequency sub-signal A 1 generated at the first level of decomposition. c The low-frequency sub-signal A 2 generated at the second level of decomposition. d The low-frequency sub-signal A 3 generated at the third level of decomposition